DESCRIPTION (Investigator's Abstract): The long term goal of this project is to elucidate the biochemical basis of the muscle atrophy that occurs during aging in humans. The immediate goal is to test the hypothesis that the rate of muscle protein synthesis declines with aging in humans. Studies will be done in fed and fasting subjects to determine if the effect of age depends on nutritional status. Both myofibrillar and sarcoplasmic protein synthesis will be determined. Muscle protein synthesis will be correlated with muscle mass strength, and muscle fiber size to determine if older subjects with the most evidence of a decline in muscle function have the lowest rates of protein synthesis. Subjects will be healthy men, both young (18-30 years old) and old (60-70 years old). Their quadriceps strength will be determined by the force of a maximal voluntary contraction, and their total muscle mass will be estimated from urinary creatinine excretion on a meat-free diet. Muscle fiber size and distribution (type 1 vs type 2) will be determined by histochemical analysis of a biopsy taken from a quadriceps muscle (vastus lateralis). Muscle protein synthesis will be determined from the rate of incorporation of 1-[1-13C] leucine into myofibrillar and sarcoplasmic proteins (obtained by biopsy of vastus lateralis) during continuous infusion of 1-[1-13C} leucine. Plasma enrichment of [13C] alphaketoisocaproate (derived from intracellular deamination of [13C] leucine) will be used to estimate the labeling of the pool of leucine used for protein synthesis. Protein synthesis also will be calculated using plasma [13C] leucine enrichment and intracellular free leucine enrichment in muscle as estimates of the labeling of the pool of leucine used for protein synthesis. The applicants suggest that these data will provide a substantial advance in our knowledge about the effect of age on protein metabolism in humans, which consists mainly of estimates of whole-body protein turnover.